The invention relates to an energy-absorbing connecting strut designed to link two components and capable of undergoing axial stresses in tension/compression between these components, so as to absorb energy in case of shock or impact on at least one of the components linked by the strut and which would be such as to develop in the latter a compressive load greater than a specified threshold, so that the shock or impact energy is not wholly transmitted by the connecting strut from one to the other of the two components which it links, and preferably so as to limit the amplitude of the force transmitted.
As an application for which the connecting strut according to the invention is of great relevance for the applicant, the invention also relates to the use of this connecting strut as a suspension strut for a main gearbox on the structure of a rotary wing aircraft, and preferably a helicopter, so that hereafter the invention is described and explained more particularly in this application.
More precisely, the energy-absorbing connecting strut according to the invention is described in its application as a so-called crash-resistant suspension strut, as it is designed to fulfil its functions of absorbing energy and limiting transmitted loads in case of a rotary wing aircraft crash, it being understood that the field of application of such an energy-absorbing connecting strut is not limited to the protection of rotary wing aircraft or at least a part of the latter, in the case of a crash.
In general and schematic terms, when a body strikes the ground, it is subjected to inertia forces which are a function of the kinetic energy accumulated by this body prior to the impact and of its deformation caused by the impact, according to the following formula:       1    ⁢          /        ⁢    2    ⁢          xe2x80x83        ⁢    m    ⁢          xe2x80x83        ⁢          (                        V          2                -                  Vo          2                    )        =            ∫      o      D        ⁢                  F        ⁡                  (          x          )                    ⁢              xe2x80x83            ⁢              ⅆ        x            
where xc2xd mVo2 is the kinetic energy of the body prior to the impact, xc2xd mV2 is the kinetic energy of the body after the impact, this energy being zero after an aircraft has crashed, F(x) represents the inertia forces applied to the body, and D is the deformation of the body caused by the impact.
From this general formula, it can be deduced that the inertia forces F(x) are greater the smaller the deformation D of the body.
If we consider the crash of a rotary wing aircraft, particularly a helicopter, in order to ensure the survival of the crew and passengers in the rotary wing aircraft, it is necessary to preserve the volume of the cabin of the rotary wing aircraft to prevent the persons occupying it from being crushed, to limit the deceleration undergone by the crew and passengers to a tolerable level and to preserve the integrity of the fuel tanks in order to prevent a fire or explosion.
In case of impact of a rotary wing aircraft with the ground, the cabin of the rotary wing aircraft is subjected to forces introduced in particular by the landing gear, the contact of the structure of the rotary wing aircraft with the ground, and all the mechanical items attached to the top of the rotary wing aircraft structure such as, in the case of a helicopter for example, the elements of the power unit, the main rotor or rotors and the main gearbox or gearboxes.
In fact, it is known that on state-of-the-art rotary wing aircrafts, and in particular helicopters, the so-called upper mechanical assemblies, namely the engines, rotors and main gearboxes are linked to the structure of the rotary wing aircraft by being bolted directly onto this structure or by a connecting device comprising a set of at least three rigid, non-deforming, straight and inclined suspension struts distributed around the gearbox and tilted so as to converge towards each other at their upper ends by which each strut is connected in a hinged manner to the gearbox, while at its lower end each strut is connected in a hinged manner to the structure of the rotary wing aircraft.
Generally, each suspension strut is hinged at its upper end directly to the main gearbox or, as a variant, to a lever supporting a flapping mass resonator and itself mounted pivotably on the main gearbox, as described for example in U.S. Pat. No. 5,190,244 and U.S. Pat. No. 6,145,785 and, at its lower end, either directly to the structure of the rotary wing aircraft, as described in FR 2 232 481, EP 718 187 and U.S. Pat. No. 5,782,430, or to a lever supporting a flapping mass resonator and itself mounted pivotably on the structure of the rotary wing aircraft, as described in U.S. Pat. No. 4,431,148, U.S. Pat. No. 4,458,862, U.S. Pat. No. 4,720,060, FR 2 777 861, FR 2 787 762 and FR 2 795 386, to which reference should be made for further details.
Currently, protective crash-resistant measures adopted on helicopters are intended to allow the absorption of energy by the landing gear, to limit the forces introduced into the helicopter structure, absorption of energy by the part of the structure under the cabin, known as the subfloor structure, to limit the forces introduced into the cabin structure, and dimensioning of the cabin structure to withstand being crushed by the upper mechanical assemblies mentioned above, linked to this structure by non-deforming means, particularly the suspension struts mentioned above.
In fact, when a crash occurs, the inertia forces originating from said upper mechanical items are very great, because of the weight of these items and the rigidity of their connection to the cabin structure.
If it is wished to preserve the volume of the cabin to prevent its occupants being crushed, the initial dimensioning of the structure, to withstand normal flying loads, is not sufficient. It is necessary to over-dimension the structure in order for it to withstand the loads during the crash, which in practice means that this structure is made very appreciably heavier.
A purpose of the invention is to propose an energy-absorbing connecting strut, the use of which as a suspension strut for a main gearbox on a rotary wing aircraft structure, as part of a crash-resistant connecting device, allows the volume of the rotary wing aircraft cabin to be preserved in the event of a crash, due to the fact that the upper mechanical assemblies can be linked to the cabin of the rotary wing aircraft by means of such connecting struts absorbing the kinetic energy of these upper mechanical assemblies, and preferably also limiting the amplitude of the forces transmitted to the cabin.
Moreover, another purpose of the invention is to propose an energy-absorbing connecting strut which, when it is used to constitute a crash-resistant device, protecting the cabin of a rotary wing aircraft from crushing by the upper mechanical items mentioned above, simultaneously provides a remedy for a number of disadvantages of known crash-resistant devices, such as presented below.
The function of all these known crash-resistant devices is to absorb energy, represented by the product of the load by the deformation.
To limit the load transmitted to a structure and which constitutes a danger of damage to the structure, it is necessary to allow a certain deformation, and known crash-resistant devices introducing deformation are of two types:
one type with elastic deformation of at least one connecting component, and
one type with plastic deformation of at least one connecting component.
The main disadvantages of known elastic deformation devices, comprising any spring system, are that they do not dissipate a substantial proportion of the energy which they receive, since they store this energy and then return the greater part of it, which results in practice in a rebound after the initial impact, which is thus followed by a succession of secondary impacts on components already weakened by the initial impact. Moreover, compared with a device absorbing energy by plastic deformation, the amount of travel required to absorb the same quantity of energy in an elastic deformation device is about twice as great because of the difference in the areas below the curves which in both cases express load as a function of deformation, these areas being representative of the absorbed energies. Such a large amount of travel is not always compatible with the size constraints for the energy absorption device.
Concerning known devices absorbing energy by plastic deformation, those proposed in WO 97/28983 for fitting to vehicle seats are triangular-braced devices comprising a telescopic load limiting rod and a tension rod deforming by necking. Though such devices are suitable for supporting seats, the weight of which is limited, they are not suitable for absorbing the high energy levels transmitted by the upper mechanical items to the structure of a rotary wing aircraft cabin in case of a crash, since then the weight of such devices and their size would be considerable.
First generation crash-resistant seats for helicopters were equipped with devices absorbing energy by plastic deformation comprising elastomer block systems cooperating with a punch, or crushable ball systems, the performance of which is inadequate because of the small amount of energy absorbable per unit volume of the plastically deformed material to be usable for protecting a rotary wing aircraft cabin against being crushed by the upper mechanical components.
These aims are achieved by means of an energy-absorbing connecting strut according to the invention, which comprises a substantially straight rigid body having at each of its two axial ends a connector for connecting respectively to one of the two components that the strut is designed to link, wherein said body comprises at least one buckling portion with calibrated buckling corresponding to a compressive load threshold, at least one hollowed portion housing at least one component absorbing energy by plastic deformation in compression, and at least one piston, facing said energy-absorbing component in said hollowed portion, and moving integrally with a rigid axial end part of said body, so that under a compressive load greater than said compression threshold of said at least one buckling portion, said buckling portion deforms causing axial shortening of said connecting strut, and movement of said piston with said rigid axial end part of the body, so that the piston crushes and plastically deforms said energy-absorbing component.
It is thus possible, with the energy-absorbing connecting strut, to reduce and control the level of force introduced by one of the two components which it links to the other.
In order to prevent any risk of tilting of the connecting strut as it buckles, it is advantageous that said piston should be guided substantially axially in the course of said buckling by a guide mechanism in the body of the strut.
In an advantageously simple and economical mode of embodiment, said guide mechanism for the piston comprises a rigid rod linking the piston to said rigid axial end part of the body, and extending substantially axially into a tubular part of the body, so as to guide substantially axially the movements of said piston with respect to said energy-absorbing component.
In order to limit the axial size of the strut, said tubular part of the body guiding said rigid rod linked to the piston advantageously constitutes at least partially said buckling portion.
This buckling portion may be embodied in any manner known to a person skilled in the art, and may advantageously be bounded by at least one localised reduction in the thickness of the wall of the body.
In an advantageously simple and economic manner, while permitting satisfactory dimensioning of the connecting strut in order to comply with the desired buckling load within an imposed size, said localised reduction in thickness is advantageously constituted by at least one of the following means: notch, groove, slot, score, substantially axial, corrugated axial section and hole in the wall of the body.
In addition, or alternatively, said buckling portion may be constituted at least partially by a material differing from that constituting the rest of the body of the strut, and/or which has at least locally undergone treatment (in particular metallurgical), and/or may present a geometry appropriate to initiating and localising the buckling.
In general terms, the buckling portion may be at least partially constituted of a material which has undergone particular treatment, making its characteristics different from those of the rest of the body of the strut.
Also advantageously, said at least one energy-absorbing component has substantially constant-load crushing characteristics over the greater part of the travel as buckling proceeds, so that the connecting strut limits the load amplitude.
In general, said energy-absorbing component may comprise at least one elastomer material as proposed for example in EP 110 233, and/or a volume of a fluid, preferably viscous, and/or a composite material, such as proposed for example in EP 322 979, or again comprising at least one organic material and/or at least one ceramic and/or at least one metal material and/or mineral or organic reinforcing fibres, with high strength characteristics.
Nevertheless, in order that the overall amount of energy absorbed should be substantial for an energy-absorbing component of small size, the latter advantageously comprises at least one honeycomb structure element, the contiguous cells of which are aligned substantially axially in said body, and preferably metal or composite, the additional advantage of such an energy-absorbing material being that it also allows the load to be limited in amplitude and to be maintained at a substantially constant level during crushing.
Preferably, said hollowed portion of the body is another hollow or tubular part of this body, which encloses said at least one energy-absorbing component, and this other hollow or tubular part of the body may be an enlarged part which is bounded, at the end opposite the piston, by a base, integral with the other rigid end part of the body, and against which said energy-absorbing component bears.
An energy-absorbing connecting strut of this kind lends itself advantageously to application as a crash-resistant main gearbox suspension strut on the structure of a rotary wing aircraft such as a helicopter, and being designed to be linked in a hinged manner, at one end to said gearbox and at its other end to the structure of the rotary wing aircraft, so that by introducing energy absorption into the connection between the structure and the main gearbox, the connecting strut according to the invention enables the level of load introduced by the upper mechanical assemblies into the structure of the rotary wing aircraft in case of a crash to be reduced and controlled. By means of suspension struts constituted by energy-absorbing connecting struts according to the invention, it is possible to dimension a connecting device between the main gearbox and structure of the rotary wing aircraft which is able to absorb the required amount of energy while complying with the desired buckling load within an imposed size.